Visual graphic display of conventional time domain analyses generally take one of two forms. The first is an oscilloscope, which may receive a time domain input signal and display corresponding time domain spectra. However, the oscilloscope does not provide a real-time, gap free representation. Rather, data that occurs after the end of an acquisition, initiated in response to an initial trigger, and before the next trigger is lost. Therefore, although oscilloscopes may be able to provide a density histogram display, the display is not gap free. Also, oscilloscopes typically lack an arbitrary sample rate that a digitizer would need to make such a gap-free feature practical to use.
The second is a real-time spectrum analyzer (RTSA), which generally receives a time domain input signal and displays corresponding frequency domain spectra, where each frequency domain spectrum represents a corresponding time interval. The spectra may be obtained by performing fast Fourier transforms (FFTs) on digital time domain data representing the time domain input signal. A conventional RTSA may also provide a power versus time (PvT) display, which is real-time and gap free. However, the PvT display aggregates multiple sequential time domain samples into PvT display points. Therefore the ability to see each individual time domain sample is lost, and the effective time resolution of the PvT display is degraded. Conventional PvT displays that use density histograms are available, but only in the context of batch mode, post processing operations that are not real-time and not gap free. An RTSA has been produced capable of providing a continuous gap free time domain histogram display, but because it lacks the ability to sample at an arbitrary sample rate, it cannot keep a stable image of an arbitrary frequency signal on the display. Rather, it must either use level triggering which means gaps are introduced, or provide an unstable display.
In addition, time domain triggering in conventional oscilloscopes and RTSAs generally is level based, where the user specifies the level (with or without hysteresis). When the signal transitions through the specified level, a trigger is produced. The trigger may be used for a variety of purposes, such as causing measurement data to be stored to memory and/or displayed on a display unit. However, when a feature of interest of the input signal is surrounded by other features that also transition through the specified level, there is no way to trigger only on the feature of interest. For example, conventional frequency mask triggering techniques permit triggering on small signals in the presence of large signals, but since the user interface is frequency domain based, specifying a time domain feature for triggering is not practical. Correlation based triggering techniques may also respond to similar shaped features in the input signal independent of the specified level of those features, thus making it difficult to isolate a particular feature. The output of a correlation trigger is ambiguous and can be difficult to interpret precisely. Frequency selective triggering, like frequency mask triggering, allows for triggering on small signals in the presence of large signals, but only when the signal of interest is isolated in the frequency domain via filtering selected by a frequency domain based interface. Within the selected frequency range, frequency selective triggering suffers from the same level based limitations in the time domain described above. Frequency mask gating allows display of only the frequency domain features of interest within a real-time continuous gap free measurement process, but otherwise lacks the ability to do so for time domain features. In those cases where level based triggering is suitable, data is lost between the end of one acquisition and the next trigger.